Marine Biology

a) Productivity in Inshore and Coastal Waters

Productivity in Inshore and Coastal Waters

The previously discussed latitudinal variations in phytoplankton productivity apply to open ocean areas away from the influence of landmasses. The situation in the water masses adjacent to land is somewhat different. There are several factors that contribute to this difference.

First, inshore waters tend to receive a considerable input of the critical nutrients, PO₄^-3 and No₃^-1, due to runoff from the adjacent land (where, as we have seen, the nutrients are far more abundant). Because of this input, inshore waters usually do not show nutrient depletion. A second factor contributing to the difference is the water depth. Most inshore waters are shallower than the critical depth; thus, the phytoplankton cannot be carried below this depth in any kind of weather. Given sufficient light, production can occur at any time, even in the winter. A third factor is that shallow inshore waters rarely have a persistent thermocline; hence, no nutrients are locked up in bottom waters. A final influencing factor is the presence of large amounts of terrigenous debris in the water, which may act to restrict depth of the photic zone and counteract the high nutrient concentration and shallow depth.

Interaction of these factors on a latitudinal basis produces changes, both in the cycle of productivity and in the total production when compared to offshore areas. In temperate regions, instead of a bimodal production cycle, as seen offshore, production remains high all through the summer. Nutrients are not limiting due to runoff from land and lack of a permanent thermocline. Yearly average production in inshore temperate waters is higher than in offshore waters due to the greater nutrient concentrations and lack of critical depth problems. The production is not even higher inshore probably is due to the presence of large amounts of light-absorbing debris in shallow water, and the fact that in offshore water, production can occur to a greater depth. In other words, in shallow waters production is limited to the upper 5-10 m, whereas offshore it may go as deep as 50 m. In tropical waters, the difference between inshore and offshore waters is particularly dramatic. Inshore tropical waters have a productivity as much as ten times that of offshore waters. This must be attributed in large par to the increased nutrient concentration inshore compared with offshore areas.

Inshore production is further enhanced through contributions of benthic plants, a component not present in offshore areas. The production of benthic algae and seagrasses is considered in Chapter 5 but is mentioned here for completeness.

Whereas we may have higher productivity in inshore or neritic waters, we also have greater variability over time and space due to local geography, river and stream discharge, storms, and tides. As a result, it is more difficult to predict the yearly productivity.